1. Field of the Invention
The present invention relates to a nickel-hydrogen battery cell which is employed in a space traveling object, such as an artificial satellite, and used as a power source.
2. Description of the Related Art
As is well known, a nickel-hydrogen battery is made up of a number of nickel-hydrogen battery cells. In each nickel-hydrogen battery cell, a nickel (Ni) electrode, which utilizes the technology of a nickel-cadmium battery, is used as a positive electrode, and a hydrogen (H.sub.2) electrode (catalytic electrode), which utilizes the technology of a known oxygen-hydrogen fuel cell, is used as a negative electrode. These electrodes are arranged in combination, and the chemical action between the hydrogen and nickel electrodes is utilized to generate desired electric energy.
The nickel-hydrogen battery cell will be described in more detail with reference to FIG. 6. As is shown, a nickel electrode 10 and a hydrogen electrode 11 are arranged such that a separator 12 impregnated with an electrolytic solution (an alkali solution) which permits movement of OH.sup.- ions is interposed therebetween. A number of electrode pairs, each electrode pair having this structure, are combined together to form a power generating element 13. This power generating element 13 is sealed inside a pressure vessel 14 formed of a metallic material, such as inconel 71. The nickel electrodes 10 of the power generating element 13 are connected to a positive terminal 15, and the hydrogen electrodes 11 are connected to an negative terminal 16 by way of harness 17. Owing to the chemical action between the nickel and hydrogen electrodes 10 and 11, the power generating element 13 generates electric energy. When the power generating element 13 is overcharged, an oxygen gas is generated from its nickel electrodes 10, due to the following chemical reaction: EQU 2OH.sup.- .fwdarw.1/2 O.sub.2 +H.sub.2 O+2e.sup.-
This oxygen gas combines with the hydrogen gas irreversibly generated from the hydrogen electrodes in the presence of catalyst, thereby forming water, as follows: EQU H.sub.2 +1/2 O.sub.2 .fwdarw.H.sub.2 O
In this type of nickel-hydrogen battery cell, an electrochemical reaction which converts the hydrogen gas generated at the time of charging into electric energy at the time of discharging, is made to take place so as to generate desired electric energy. Since such an electrochemical reaction is utilized, the nickel-hydrogen battery cell will deteriorate in characteristics if the ambient temperature is low.
To solve this problem, the measures shown in FIG. 7 are taken. As shown, a sleeve member 19, formed of an aluminum alloy, is coupled to the outer periphery of the pressure vessel 14 by use of a silicone-based adhesive improved in thermal conductivity, in such a manner that the sleeve member 19 surrounds the pressure vessel 14. A sheet heaters 1 are adhered to the outer periphery of the sleeve member 19, for temperature control. When the sheet heaters are selectively driven, the heat generated thereby is transmitted to the pressure vessel through the respective sleeve members 19, thereby controlling the pressure vessel 14 at a temperature suitable for electrochemical reaction.
In the nickel-hydrogen battery cell mentioned above, however, the sleeve member 19 is heated by the sheet heaters 1, and the temperature of the pressure vessel 14 is controlled with the heat transmitted through the sleeve member 19. Due to this structure, the battery cell is poor in heat control characteristics, and the sheet heaters must be large in capacity.
In addition, the operation of adhering a sheet heater 1 to a sleeve member 19 and coupling this sleeve member 19 to a fixing plate 18 has to be performed for each of the nickel-hydrogen battery cells. Therefore, the assembling operation involved in the manufacture of the nickel-hydrogen battery is very complicated.
These are very serious problems in the field of space development since saving power is particularly important in the field.